Turbines arranged to extract energy from a streaming fluid—such as wind, steam, tidal streams and water waves—are usually optimized to reach maximum efficiency at a certain fluid speed, that is designed for a certain operating point. Basically, turbine designs assume a fluid velocity is present to give a lifting force on the rotor blades and so overcome the total drag and friction forces of the rotor and turbine in operation. Thereby, the turbine axis will rotate due to the moment produced by the lifting force that corresponds to the product of the second order pressure gradient of the fluid passing over the wing profile of the rotor blades times the distance to the axis of turbine rotation. This applies for conventional wind turbines, steam turbines and tidal turbines having an axis of rotation parallel to the fluid, that is a horizontal orientation; and for unconventional transversal turbines having an axis of rotation at right angles to the fluid, for example vertical axis of type H-rotor SE564997C2 or horizontal axis of type US2011/110779A1.
Transversal turbines are known to exhibit an axis of turbine rotation and a rotor which revolves around said axis, and four significant fluid passages through the rotor: windward, downstream, leeward and upstream. The result is a lifting force vector on the rotor blades that pulsates during each complete turn of the rotor, so the absolute value of the pulse fluctuates by the cosine; to show maximum values to the windward and leeward and minimum values downstream and upstream. The rotation of the rotor is known to be transferred to the turbine by one or more joints, each one of which is exhibiting an attachment point shared with a rotor blade. Thus, the rotor and rotor blades may be joined to the turbine in several attachment points. In the said H-rotor design, the rotor is mounted to the turbine in two attachment points located symmetrically around the mid-normal plane of the rotor, that is centre supported; while the Darreus wind turbine NL19181 has a rotor supported in both ends by the turbine, that is double-end supported. Also known is a rotor supported in one end only, that is single-end supported; that is provided with straight rotor blades having an oblique axis of turbine rotation and joined to a central hub at the ground level. It is further known that centre supported turbines are provided with a supporting framework of supporting arms, hub and turbine axis; all of which are disturbing the flow pattern when the fluid passes through the blade body of the rotor, so the lifting force decreases on the rotor blades; particularly to leeward, which reduces the turbine efficiency. This disadvantageous effect of transversal turbines reduces the efficiency, as compared to conventional turbines having a horizontal axis of turbine rotation and the same solidity; that is the ratio of the rotor blade tip velocity over fluid velocity is decreased. For this reason, investments in wind power plants prefer turbines of a horizontal axis compared to transversal. Thus, as it may be realized, there is a need for a rotor that is not centre supported in order to increase the efficiency of transversal turbines.
Transversal turbines are known to equip preferably three straight rotor blades, for example the H-rotor design having blades extending parallel to the axis of turbine rotation. It is further known that a centrifugal force arises due to the rotation, acting on the turbine mass and directed outwards at a right angle to the axis of rotation. Thus, the centrifugal force vector adds to the lifting force vector on the rotor blades; composing an additional radial force vector on the rotor blades at the downstream, leeward and upstream passages, but a reversed radial force vector windward. Thus, the resulting pulsating force vector reaches its maximum at leeward and minimum at windward; which difference increases the risk of material fatigue, critical vibration resonance modes and undesired noise from the turbine rotation. Rotors provided with straight rotor blades exhibit these design challenges which have to be overcome at the design of attachments to the rotor blades. It is further known that transversal turbines having three straight rotor blades cannot start by themselves at low fluid velocities, but by the generator; while helical rotor blades are easier to start. Rotor blades of helical shape are known, amongst other from CA 2674997 which describes a wind turbine having helical rotor blades arranged vertically and attached directly at multiple locations to the hub on the axis of the turbine. Also, in WO2120153813(A1) a water turbine is described in analogy with the said Darreus turbine NL19181 but provided with multiple rotor blades; and in WO2120152869(A1) which describes a water turbine of foldable rotor blades of helical shape. Thus, as it may be realized, a rotor provided with helical rotor blades exhibits advantages which can be further emphasized and developed.